1. Field of the Invention
The invention relates generally to a direct access storage device (DASD) of the type utilizing partial-response maximum-likelihood (PRML) detection, and more particularly to a method and apparatus for asynchronous gain adjustment for PRML data detection.
2. Description of the Prior Art
Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks.
In a PRML data channel, a normalized readback signal amplitude is required for proper data detection. A variable gain amplifier (VGA) typically is used in the analog signal path for scaling of the readback signal. Known PRML channels require an analog envelope detector circuit to sense the amplitude of the incoming readback signal in order to provide gain corrections to the VGA in the idle mode before the PRML channel begins a read or write operation. An acceptable tolerance on the signal amplitude must be maintained during the idle mode in order to guarantee fast convergence at the start of a read operation. Disadvantages of using an analog envelope detector circuit include the usually poor amplitude tolerance without on-chip adjustments because the circuit is analog. Also the analog envelope detector circuit is difficult to build for high speed signals without increasing the circuit power significantly. In addition, recovery from a gain being too low is typically very poor.